1. Field of the Invention
This invention relates to a power management system and more particularly to a power management system for the supply of power to an implantable medical device, such as an implantable hearing prosthesis.
2. Related Art
Implantable medical devices, such as cochlear implants, middle ear implants, FES systems and the like, typically consist of two components, one part being an external component commonly referred to as a processor unit and the other part being an implanted internal component commonly referred to as a stimulator/receiver unit. Traditionally both of these components cooperate together to provide a desirable therapy to the implantee. In the case of implantable hearing prosthesis such as cochlear implants and/or middle ear implants, the external component has consisted of a microphone for detecting sounds, such as speech and environmental sounds, a speech processor that converts the detected sounds and particularly speech into a coded signal, a power source such as a battery and an external antenna/transmitter.
The coded signal output by the speech processor can be transmitted transcutaneously to the implanted stimulator/receiver unit situated within the head of the implantee. The transmission can occur through use of an inductive coupling provided between the external antenna transmitter and an implanted antenna/receiver which forms part of the stimulator/receiver unit. The communication serves to transmit the coded sound signal and to provide power to the implanted stimulator/receiver unit. The external part is generally worn outside the skin and can be positioned behind-the-ear like a traditional BTE hearing aid and contains the components mentioned previously.
The implanted stimulator/receiver unit typically includes the antenna/receiver that receives the coded signal and power from the external processor component, and a stimulator that processes the coded signal and outputs a stimulation signal to an assembly, which applies the stimulation to generate the desired therapy. In the case of a middle ear implant, the assembly may include a mechanical or hydromechanical actuator device that is coupled to the ossicles of the middle ear or directly to the inner ear for applying direct stimulation thereto, producing a hearing sensation corresponding to the originally detected sound. In the case of cochlear implants, the assembly may include an intra cochlear electrode assembly, which applies electrical stimulation directly to the auditory nerve producing a hearing sensation corresponding to the original detected sound. The implanted unit is located under the skin inside the mastoid and contains primarily means to demodulate or decode the signals transferred through the skin to drive an actuator or an electrode array and to convert the power transferred through the skin into an electric supply voltage. In known systems the implanted portion does not contain any independent power source and consequently the transcutaneous link must be in place and therefore the wearer must wear the external component permanently, which impedes the comfort of the wearer.
Due to other particular problems with the external components of existing hearing prosthetic devices, such as the aesthetics associated with wearing a visible external device, the wearer having to remove the device while showering or engaging in water related activities, and the likelihood that the alignment between the external and internal coils will be lost due to movements during sleep or physical activity, there exists a need to provide a system that allows for total freedom with improved simplicity and reliability. Consequently research has been directed to fully implantable hearing prosthetic systems that do not require external components for the operation thereof. This would include providing a medium-to-long term power source such as rechargeable batteries, in the implants to overcome the need for power to be continually transmitted to the implant from an external power source.
However certain challenges appear with providing implanted devices with such implanted power sources. Unlike ceramic capacitors used as present-day short-term energy reservoirs, a rechargeable battery degrades irreversibly during its lifetime. They can only undergo a particular maximum number of recharging cycles before the battery performance diminishes to a level where the battery is essentially unusable. The degradation is due to the decomposition of the electrolyte and due to electrode corrosion. The decomposition of the electrolyte can cause the generation of side products like hydrogen and other gases that tend to build up pressure in the battery enclosure and finally escape through flaws in the enclosure. Corrosion of electrodes leads to a reduction of the active surface area and may ultimately, if the battery continues to be charged and discharged, cause a short circuit inside the battery through metal spikes growing from one electrode to the other. Both effects lead to a reduction of the available battery capacity and ultimately to total failure. In the worst case, the implanted battery may rupture causing a severe threat to the health and well being of the wearer. Furthermore, if rechargeable batteries are discharged before being fully charged or conversely charged before being fully discharged, their overall capacity may be prematurely reduced.
Totally implantable hearing systems require a substantial amount of energy to be stored since the whole signal processing unit and supporting functions have to be supplied for a reasonable period of time. In order to be commercially viable, the required battery life per charge is generally considered as being in the order of a week. On the other hand, the number of recharging cycles of a storage battery is limited. Thus the capacity and ultimate size of an implanted battery must be made as large as possible. However, such application of large batteries causes further challenges, including that they require thick enclosures, that is considered dead space, in order to retain the pressure of evading gases. Large batteries are also prone to the occurrence of internal short circuiting and battery failures are due to their larger surface area. Furthermore, the larger the battery is, the more severe the thermal effect of a battery failure causing accumulative thermal damage to surrounding tissue.
It is desired to overcome, or at lease ameliorate any one or more of the shortcomings of prior arrangements.